The present invention relates to an ultrasonic probe and a manufacture method for same, and more particularly to an ultrasonic probe suitable for use in an apparatus which utilizes high-frequency sound energy, such as an ultrasonic microscope, and a manufacture method for the probe.
In view of the fact that ultrasonic waves with their frequency as high as 1 GHz have wavelengths in the order of about 1 .mu.m in water, ultrasonic microscopes have been fabricated by utilizing signals caused by disturbances such as reflection, scattering, and attenuated transmission. A ultrasonic probe equipped with an acoustic lens is employed as means for condensing an ultrasonic beam onto the objective to be measured. The ultrasonic lens comprises a crystal such as sapphire, quartz glass, or the like, and is configured to have a spherical lens surface on one side and a flat surface on the other side. On the flat surface side, there is disposed a piezoelectric transducer for radiating RF ultrasonic waves in the form of plane waves. The plane waves radiated from the piezoelectric transducer propagate through a lens body, and are then condensed to a certain focus by a positive lens surface that is consitituted by the interface between the spherical lens surface and a medium (e.g., water).
To prevent attenuation of the ultrasonic waves while propagating from the lens surface to the focus through the medium, the distance from the lens surface to the focus should be as short as possible. On the other hand, it is required for increasing resolution that the F-number of lens (i.e., the ratio of focus distance to aperture of the lens surface) be sufficiently small. Therefore, the lens surface must be a minute spherical surface with diameter in order of 200 .mu.m. In addition, the lens surface must be free of any unevenness of size larger than 1/10 the ultrasonic wavelength. This size is in order of 0.1 .mu.m when using the ultrasonic waves of 1 GHz.
To date, such an acoustic lens has exclusively been machined by a mechanical grinding technique. From a practical point of view, however, the spherical surface with diameter less than 500 .mu.m could hardly be formed by the mechanical grinding technique. In order to overcome that difficulty, there has been proposed a method of solidifying the surroundings of air bubbles produced in molten glass, and then machining the half surrounding surface of a desired air bubble (JP. A. 58-4197), or a method of pressing a spherical glass ball against a glassy carbon material before sintering, to thereby form a recessed surface, and then sintering the carbon material (JP. A. 59-93495).
However, the method of exploiting air bubbles in the glass has a difficulty in finding out the desired air bubble of proper size. Even if the desired air bubble is found out, it could not be used in practice if any other air bubbles are present in the vicinity thereof. Thus, the proposed method is not likely to become established as a lens manufacture method for industrial purpose. Also, it will be apparent that this type method cannot provide a lens surface (e.g., cylindrical surface) of the shape other than spherical.
Meanwhile, the method of pressing a glass ball against a glassy carbon material and then sintering the latter has several problems that non-negligible scattering of ultrasonic waves are caused due to the presence of air bubbles or inclusions remaining in the sintered material, and sintering causes a substantial change in size.
Further, the outer edge of the lens surface is usually ground into a tapered shape to keep the unnecessary reflected waves from being received. Observing the ground portion in large magnification, the flat surface is left between the lens surface and the tapered surface. If the tapered surface is machined to an extent that eliminates the flat surface completely, the edge of the lens surface would be chipped off or made somewhat round. In either case, therefore, the noise received through the outer peripheral portion cannot be reduced.
In addition, it becomes feasible to capture a two-dimensional image of the objective to be measured, by densely arranging a number of spherical lenses on a flat surface (JP. A. 56-103327). Also, sound image information could be obtained from multiple points simultaneously if a plurality of lens surfaces can be arrayed on a flat surface with high precision. With the mechanical grinding method and the method of finding out air bubbles in glass, however, it is practically impossible to array a plurality of lens surfaces on a single substrate with high precision. The sintering method cannot avoid some fluctuations in the pitch of lens array concomitant with the sintering step. Moreover, extreme difficulties are encountered in creating an array of lens surfaces by combining many individual single lenses, taking into account the minute lens size.
As described above, the prior art has accompanied the problems of extreme difficulties in machining the lens surface of minute curvature with high precision, and of very expensive acoustic lenses. Another problem was a limitation encountered in reducing the noise received through the outer peripheral portion of the lens surface. Still another problem was in that infeasibility or extreme difficulties were found in obtaining a two-dimensional information of the objective to be measured or obtaining sound image information from multiple points simultaneously by arraying a plurality of lenses on a flat surface with high density and/or high precision.
It is an object of the present invention to provide an ultrasonic probe equipped with an acoustic lens which has a lens surface of the very small radius of curvature and can be fabricated inexpensively, and a manufacture method for the ultrasonic probe.
Another object of the present invention is to provide an ultrasonic probe equipped with an acoustic lens which can reduce the noise received through the outer peripheral portion of the lens surface, and a manufacture method for the ultrasonic probe.
Still another object of the present invention is to provide an ultrasonic probe equipped with an acoustic lens which comprises a plurality of minute lenses arrayed with high density and/or high precision, and a manufacture method for the ultrasonic probe.